The ability to reduce the surface tension of water is of great importance in waterborne coatings, inks, adhesives, fountain solutions, agricultural formulations, and other industrially important formulations because decreased surface tension translates to enhanced substrate wetting in actual formulations. Surface tension reduction in water-based systems is generally achieved through the addition of surfactants. Performance attributes resulting from the addition of surfactants include enhanced surface coverage, fewer defects, and more uniform distribution. Equilibrium surface tension (EST) performance is important when the system is at rest. However, the ability to reduce surface tension under dynamic conditions is of great importance in applications where high surface creation rates are utilized. Such applications include spraying, rolling and brushing of coatings or spraying of agricultural formulations, or high speed gravure or ink-jet printing. Dynamic surface tension (DST) is a fundamental quantity which provides a measure of the ability of a surfactant to reduce surface tension and provide wetting under such high speed application conditions.
Traditional nonionic surfactants such as alkylphenol or alcohol ethoxylates, and ethylene oxide (EO)/propylene oxide (PO) copolymers have excellent equilibrium surface tension performance but are generally characterized as having poor dynamic surface tension reduction. In contrast, certain anionic surfactants such as sodium dialkyl sulfosuccinates can provide good dynamic results, but these are very foamy and impart water sensitivity to the finished coating.
There is a need for surfactants which exhibit good equilibrium and especially good dynamic surface tension properties, are preferably low-foaming, and would be widely accepted in the waterborne coating, ink, adhesive, fountain solution and agricultural formulation industries.
The importance of reducing equilibrium and dynamic surface tension in applications such as coatings, inks, adhesives, fountain solutions and agricultural formulations is well-appreciated in the art as taught in the following documents:
Schwartz, J. “The Importance of Low Dynamic Surface Tension in Waterborne Coatings”, Journal of Coatings Technology, September 1992;
Wirth, W.; Storp, S.; Jacobsen, W. “Mechanisms Controlling Leaf Retention of Agricultural Spray Solutions”; Pestic. Sci. 1991, 33, 411–420; and
Medina, S. W.; Sutovich, M. N. “Using Surfactants to Formulate VOC Compliant Waterbased Inks”, Am. Ink Maker 1994, 72 (2), 32–38.
Extensive background literature exists for the measurement of low equilibrium surface tension of anionic-cationic surfactant mixtures. See “Catanionic Surfactants,” A. Khan and E. Marques in Specialist Surfactants, Published by Blackie Academic and Professional, London, pp. 37–80 (1997) and “Anionic-Cationic Surfactant Mixtures,” A. Mehreteab, Surfactant Sci. Ser., 82 (Handbook of Detergents, Part A), 133–155 (1999) for reviews.
Several patents describe applications of such anionic-cationic surfactant mixtures in detergents or shampoos, namely, U.S. Pat. Nos. 4,330,526; 4,888,119; 4,919,839; and 5,441,541. A recent patent, U.S. Pat. No. 6,010,996 describes the use of anionic-cationic surfactant mixtures of strong acid anionic/permanent quaternary cationic surfactants as biocides and surface tension reducers with good performance under dynamic conditions.
In “Effects of Structure on the Properties of Pseudononionic Complexes of Anionic and Cationic Surfactants”, A. Mehreteab in ACS Symp. Ser. 501 (Mixed Surfactant Systems) pp. 402–415 (1992), a description of lowering of dynamic surface tension (DST) by “pseudononionic” ethoxylated cationic-anionic surfactant blends was given, but DST were still high at >45 dyne/cm at 4 bubbles/sec.
In “Dynamic Surface Properties of Anionic-Cationic Mixtures”, P. Joos, J. Van Hunsel and G. Bleys, J. Phys. Chem., 90, pp. 3386–3393 (1986), some theoretical support is given that dynamic surface tension of these mixtures should be dependent on diffusion characteristics of either the two individual surfactant diffusion coefficients, which in most cases will reduce to the diffusion coefficient of the electroneutral complex.
In “Dynamic Surface Tension of the Aqueous Solutions of Cationic-Anionic Surfactant Mixtures”, L. -H. Zhang and G. -X. Zhao, J. Colloid and Interface Sci., 127 (2), 353–361 (1989), the authors describe DST reduction by anionic-cationic surfactant blends in which the only examples of complexes having DST<40 dynes/cm were fluorinated surfactants.
Finally, in “Dynamic Surface Tension of Aqueous Surfactant Solutions: 5. Mixtures of Different Charge Type Surfactants”, M. J. Rosen and T. Gao, J. Colloid Interface Sci., 173, 42–48 (1995), the authors describe anionic-cationic surfactant mixtures which have higher DST than the individual parent surfactants. The authors state that “{in anionic-cationic surfactant blends}, the strong interaction of the two components to produce a bulkier, more hydrophobic, tight ion pair containing two hydrophobic groups results in a reduction of the rate of diffusion of the surfactant in the mixtures to the interface and a consequent increase in the surface tension at short times (log t<0).” The authors go on to state that “at short times (log t<0), because of the interaction of the two components, the {dynamic} surface tension of the mixtures is again greater than that of either component by itself.”
In all of the above citations reference is made to the use of permanent quaternary (R4N+X−) cationic surfactants. There is no consideration of weak base amine (R3N) compounds which can reversibly protonate to R3NH+X− cationic species.
A number of patents have appeared on alkyl, hydroxyalkyl and hydroxyalkylether substituted weak base mono- di-, tri- and higher amine based surfactants. These include U.S. Pat. No. 6,235,820 alkylated aminoalkylpiperazine surfactants; U.S. Pat. Nos. 6,190,733; 6,051,056; alkylated aminoether surfactants; U.S. Pat. Nos. 6,015,852 and 5,939,476 surface tension reduction with alkylated higher polyamines.
Copending applications describing amine/glycidyl ether adducts for use as foam reducing components include “Alkyl Glycidyl Ether-Capped Polyamine Foam Control Agents”, Ser. No. 09/909,555 filed 20 Jul. 2001 and “Alkyl Glycidyl Ether-Capped Diamine Foam Controlling Agents”, Ser. No. 10/061,898 filed 1 Feb. 2002.